1. Field of the Invention
The present invention generally relates to electronic sensors, and more particularly to the fields of surface acoustic wave sensors and surface plasmon resonance sensors.
2. Discussion of the Related Art
There are a variety of situations in which it is desirable to idetifty a target analyte by evaluating an air sample near the object or other material. For example, canines have long been used to locate hidden drugs and identify land mines. Indeed, hidden land mines continue to pose a significant threat to civilians worldwide. As a result of wars and other armed conflicts, many millions of unexploded land mines remain buried over some 60 countries worldwide. It has even been reported by the International Committee of the Red Cross that land mines claim a victim every 20 minutes. Presently, the discovery and removal of these explosives is a very tedious and expensive process. Accordingly, rapid, and cost-effective detection is key to the timely removal of the millions of land mines already scattered worldwide. Alternatives to metal detectors include some technologies that are also being developed for detection of bombs and unexploded ordnance. Some biologically based approaches have been suggested.
More specifically, surface acoustic wave (SAW) technology has been proposed as one possible manner of more rapidly detecting land mines. As is known, when an electric field is applied to a piezoelectric material, a sound wave of specific frequency can be generated on the surface. The frequency of the surface acoustic wave varies based upon the material and its surface characteristics. Accordingly, certain objects and materials can, essentially, produce a SAW frequency signature. In this way, identification of the frequency signature identifies the target analyte. Nominally, the SAW sensor measures the mass of the target analyte.
Surface plasmon resonance (SPR) technology is another technological field that is used to identify target analytes. SPR has been known for over 20 years and is used to identify the dielectric permittivity of the target analyte. As is known, SPR is the oscillation of the plasma of free electrons which exists at a metal boundary. These oscillations are affected by the refractive index of the material adjacent the metal surface. SPR may be achieved by using the evanescent wave which is generated when a TM-polarized light beam is totally internally reflected at the boundary of a medium, e.g., glass, which has a high dielectric constant.
In general, an SPR configuration includes a source of electromagnetic radiation (light), an optically transmissive (transparent) component (the SPR sensor) which has a conducting film (e.g. a metal layer) on it, and a detector. The conducting film is in contact with a dielectric. Light is transmitted into the transparent component, undergoes total internal reflection, and if the conditions outlined in the equations above are met, then a surface plasmon wave will occur at the surface of the conducting film, that is at the interface of the metal layer and the dielectric. The detector measures the resonant phenomenon.
To illustrate, reference is made to FIG. 1, which is a diagram that illustrates a setup for evaluating SPR. A beam 1 of light is directed from a laser source (not shown) onto an internal surface 2 of a glass body 3. A detector (not shown) monitors the internally reflected beam 4. Applied to the external surface 2 of glass body 3 is a thin film 5 of metal, for example gold or silver, and applied to the film 5 is a further thin film 6 of organic material containing antibodies. A sample 7 containing antigen is brought into contact with the antibody film 6 to thus cause a reaction between the antigen and the antibody. If binding occurs, the refractive index of the film 6 will change owing to the increased size of the antibody molecules, and this change can be detected and measured using surface plasmon resonance techniques.
Sensors based on the SPR effect sense the refractive index (RI) of a thin region adjacent to the sensing surface. SPR can be applied indirectly to other sensing applications by treating or manipulating the sensing surface such that the refractive index at the surface varies with the presence of the substance to be sensed. For instance, the surface can be made sensitive to a particular antibody by coating the surface with an antigen for that antibody. When the antigen binds to the antibody, the refractive index at the surface changes slightly. A commercial application of SPR to biological sensing has been developed using this principle.
The practical effect of a change in the RI of the dielectric adjacent to the SPR sensing surface is a shift in the SPR resonance curve. If the wavelength modulation technique is being used, the resonance curve of interest is the reflected intensity of light versus the incident wavelength. The minimum of this curve is defined as .lambda..sub.sp, which is the SPR resonance minimum in wavelength space. If the angle modulation technique is being used, the resonance curve of interest is the reflected intensity of light versus the incident angle. The minimum of this curve is defined as .theta..sub.sp, which is the SPR resonance minimum in angle space. It is also possible to determine the resonance from looking at the transmitted light intensity using either of these techniques. The techniques can also be combined, in which case the three dimensional intensity-angle-wavelength space must be considered. The absorption of the dielectric layers, which is directly related to the imaginary part of the refractive index of the dielectric layers, can also be determined from the SPR resonance. More absorbing dielectric layers, such as dye indicators (for instance, methylene blue), cause broader, less deep resonances. Parameters such as the resonance depth, or the resonance width, are not used as much as the resonance minimum location, because SPR is much more sensitive to changes in the real part of the index of refraction than it is to changes in absorption.
Though SPR detection and evaluation techniques have been demonstrated and has been shown to yield a high degree of sensitivity, the required set up has proven useful only in a laboratory or highly controlled setting. The primary limitations to usage of standard SPR setups are (1) they are large and unwieldy, and cannot readily be made compact, (2) they must be carefully isolated from shock and external vibration, and (3) the cost of the entire system is relatively high. All of these effects make the standard SPR setups of little use in field portable systems or remote sensing applications.
Accordingly, there is a heretofore unaddressed need to provide a remote or field sensor configuration for detecting and/or measuring an analyte in a field setting.